The genus Xenophora is famous for attaching random, or not so random bits of rock and shell to the exterior of the shell. The specimen here seems to have used mostly small shell fragments and rocks, but other species use only bivalve shells, or project long pointed gastropod shells outwards. The Zymogyphic museum has a great collection of photos to give you the first taste of the variety and precision sometimes used by Xenophora.

How do they attach stuff? Very carefully:

“Characteristically, the shell is covered with other shells, shell fragments, coral pieces, or stones that are attached or cemented with secretions from the animal. The shells are attached dead, although there is one account of a live kitten’s paw being attached in an aquarium. All bivalves and bivalve pieces are attached inner side up and gastropods are usually attached with the aperture up. Once an object is selected, it is cleaned (as is the site of intended attachment), and then the object is rotated and fitted to the attachment site. This may take up to 1 1/2 hours. The piece is then held in place with the animal’s foot, snout, and tentacle bases and glued into place. The Xenophora may then lay motionless for up to 10 hours, only rocking in place now and then, seemingly a check on the strength of its new attachment.”

These snails feed on forams and microscopic algae, and even bury their feces – presumably to reduce chemical signals that could attract predators. The collection is mostly thought to be for camouflage: it breaks up the shell outline, and holds the shell above the substrate so the snail can feed while still remaining under the shell.

For more info, and the key I used for identification:

[1] Ponder, Winston F., 1983. A revision of the Recent Xenophoridae of the world and of the Australian fossil species (Mollusca, Gastropoda). Australian Museum Memoir 17: 1–126, with appendix by W. F. Ponder and J. Cooper. [31 December 1983].